This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2004-336145 filed in Japan on Nov. 11, 2004, the entire contents of which are hereby incorporated by reference.
The present invention relates to a method for producing a printed wiring board.
In general, a printed wiring board is produced after undergoing many processes for laminating and bonding a conductor layer and an insulating layer. In these production processes, it is a very important factor to secure the bond strength by improving the contact performance between layers of a printed wiring board.
For example, in a multilayer wiring board, an insufficient bond strength between an interlayer insulating layer and a conductor metal layer can break the wiring board or disconnect the circuits, which can lead to malfunction of an electronic device that has the wiring board mounted thereon, because the layers may separate from one another when a temperature stress or a bend stress is applied to the wiring board. It is also known, especially with a printed wiring board that has flexibility, such as flexible wiring board and flex-rigid wiring board, that the contact property and the bond performance between a conductor layer and the insulating layers that are placed above and below it not only cause a problem of blisters due to temperature and moisture stresses but also affect the bend performance of the same.
For these reasons, various measures have been proposed to improve the bond performance between layers, among other things, the bond performance in bonding different materials, such as the bond performance between a conductor layer and an insulating resin layer and the bond performance between a conductor layer and a cover lay layer.
The examples include a blackening treatment and a browning treatment in which the surface of a conductor layer is coated with an oxide film. In addition, a method for a surface roughening treatment in which a conductor metal is dissolved using a chemical prior to bonding of each of the layers so that its surface becomes rough has also been proposed. Furthermore, such methods as a polishing method in which the surface of a conductor is mechanically polished to add projections and depressions so that the bonding area becomes larger with the bare metal surface exposed, and a laser abrasion method in which the surface of a conductor is scanned with a laser beam to clean the surface of the conductor and improve the reactivity have yet also been proposed and actually been utilized. In addition, a method for treating the bonding surface with plasma has yet also been proposed as disclosed in JP 2003-163451.
As mentioned above, various methods have been proposed for improvement of the contact property and the bond strength between layers of a printed wiring board. In reality, however, they do not always result in improvement in the contact property and the bond property between the layers. They have a great impact on the bend performance especially of a flexible wiring board whose features include the use while it is bent. Since a flexible wiring board is made by bonding polyimide that is difficult to bond and a copper film, there is a reality in which the above mentioned methods may not always be able to secure a sufficient bond strength between the cover lay and the conductor pattern at the portion bended as mentioned above.
Given these factors, the applicant of the present invention focused attention on forming numerous protruding metal crystals on the surface of a conductor portion and utilizing them to secure the superior contact property and bond strength, which were impossible to achieve with the above mentioned techniques.
One example of such a projecting metal crystal is a dendrite. A method for forming this dendrite has been disclosed, for example, in JP S57-34356A and in JP 2003-298264A. In addition, an application example of a dendrite as mentioned above is disclosed in JP H1-198092A. In this example, a dendrite is utilized as an anisotropic connector, and this example has a configuration in which a dendrite is formed on a conductor pattern and it is then filled with a resin. In order to secure the conductivity between layers through this dendrite, the tip of the dendrite is exposed to the surface of a bonding layer.
In addition, in JP H7-231152A, a method for improving the contact property between layers by providing bumps on the surface of a conductor is proposed. This method is to form a rough surface for a bonding layer simply by forming very fine bumps in the shape of an inverted teardrop.
In addition, in JP H5-75233, a method for preventing a line disconnection or a line lifting by forming copper oxide and then forming a resist image on the surface through reduction of the same to copper is proposed. In addition, in JP H6-169168A, JP H7-202367A, and JP H8-236930A, a method for attempting to improve the bond strength by providing projections on the surface of a conductor is described, but the intention there is rather to inhibit generation and development of dendrites.